A Cascade of Interesting Chemistry

Zn in the +III Oxidation State

Zinc’s ground state electronic configuration is 3d104s2. As such, it can readily form complexes with an oxidation state of +2. However, higher oxidation states are unknown as they would imply that electrons from the highly stable filled 3d orbitals be involved in bonding.

Jena and Semanta report using DFT calculations to establish the possibility of a complex of zinc in the +3 oxidation state. The key to achieving this is in using ‘super halogen’ ligands. Super halogens are species with extremely high electron affinities. Here the authors use BO2 and AuF6, whose electron affinities measure as 4.5 and 8.4 eV, respectively, compared with 3.4 eV for fluorine.

The group started by first looking at the neutral ZnF3 complex. Here they found that 2 of the bonded F atoms appear very close together. As such, they may be considered quasi-molecular, with partial bonding between the the 2 fluorine atoms. The authors conclude that, in this case, each F atom can only be considered as having and oxidation state of -0.5, leaving zinc in the +2 oxidation state. It is also noted that ZnF3 would be unstable to fragmentation.

Zn(BO2)3 also was unsuccessful in achieving a Zn(III) complex; calculations showed that 2 of the ligands dimerise, leaving only 2 bound to zinc.

In Zn(AuF6)3, zinc is coordinated to 6 fluorine atoms. Zn(AuF6)3 displays a similar geometry to that of ZnF3 with 2 of the ligand species appearing close to each other. However, this complex is stable to fragmentation and ligand dimerisation, and so can be considered as a stable Zn(III) complex.

So, does this mean that zinc’s 3d electrons are being perturbed to involvement in bonding? Carrying out natural bond orbital charge distribution calculations, the authors conclude that the charge on zinc in the Zn(AuF6)3 complex is around +1.7 and that the d orbitals are completely filled. The authors discuss at length that the formalism of oxidation state indicates a charge on an atom if bonding is completely ionic, which, in covalent complexes, it is not.

So, does this complex represent a radically different kind of zinc compound? Or is it just a quirk of our formalism of ‘oxidation state’ (we call it Zn(III) but it’s bonding is not fundamentally different to Zn(II))? Honestly, I really don’t know.*